1. Field of the Invention:
The present invention relates to excitation systems for synchronous dynamoelectric machines, and more particularly to a method and means for fast de-excitation of the brushless exciter of a synchronous generator.
2. Description of the Prior Art
Brushless excitation systems are now widely used for supplying direct current field excitation to synchronous dynamoelectric machines such as large alternating current generators. Such brushless excitation systems include an alternating current exciter having a stationary field structure and a rotating armature member. A rotating rectifier assembly is carried on a common shaft with the exciter armature and is connected thereto to provide a direct current output. The output of the rectifier is connected to the field winding of the main generator which also rotates with the exciter armature and rectifier. In this way, an excitation system is provided which requires no sliding contacts.
In conventional arrangements, the main exciter for a synchronous generator comprises an alternating current generator having its armature mounted on the same shaft as the field winding of the synchronous machine and also having a stator field winding which must be energized by direct current to create a magnetic field so that a voltage will be induced in the rotating armature of the exciter. In well known arrangements, the direct current excitation for the main exciter is provided by a pilot exciter having a permanent magnet rotor turned by a prime mover within an annular armature winding to produce excitation power for the main exciter. Means such as a rectifier circuit is ordinarily provided to convert the alternating current output of the pilot exciter to direct current for the main exciter field excitation.
With use of a rotating rectifier, efficiency and reliability are achieved through the elimination of maintenance and replacement problems normally associated with brushes, commutators and collector rings. U.S. Pat. Nos. 3,549,919 and 3,705,331 are illustrative as to known brushless exciters which utilize conventional semiconductor diodes. In such arrangements, several semiconductor diodes are connected in parallel for each AC phase and as used heretofore each diode has a separate fuse element in series therewith so as to avoid a direct short between the exciter armature and the generator field upon a shorting failure of the diode, to which semiconductor diodes are susceptible. U.S. Pat. Nos. 3,341,328 and 3,671,850 are illustrative as to the use of controlled rectifiers such as thyristors in place of the conventional diodes in the rotating rectifier assembly of a brushless exciter. Thyristors have proven to be particularly suitable for rotating equipment applications, since they are relatively insensitive to vibration, extreme temperature environments, and accelerative forces. Additionally, they afford relatively fine control of the excitation so that an extremely large range of exciter current is available for both the forcing mode of operation and counter-excitation, more commonly known as fast de-excitation.
In conventional exciter arrangements, it has been the practice of the industry when applying thyristors for excitation control that the armature of the exciter is operated continuously at or near a nominal ceiling voltage, the ceiling voltage being determined by the maximum voltage rating of the rotating rectifier components. In order to vary the excitation, the firing signals to the thyristors are applied only during positive portions of the armature voltage waveforms thereby controlling the excitation to the main generator field. While this brushless excitation arrangement has been employed successfully in some cases, serious problems have emerged which are inherent in this operational mode. For example, because the thyristors are operated continuously near a nominal ceiling voltage, they must be derated to provide excitation control. Also, the exciter size must be chosen according to the continuous rating of the unit at the nominal ceiling voltage so that the conductor size is correspondingly large to accommodate operation at the nominal ceiling voltage level. As would be expected, the cooling requirements for such an arrangement are also increased to accommodate the thermal requirements of the increased mechanical and electrical losses.
Thyristors, with gate control, permit more control over the excitation as compared to the use of conventional diodes for rectification. However, as discussed above, in prior known systems, the thyristors are switched at a relatively high ceiling voltage that requires a high power rating, and additional components are required for fast de-excitation. For the foregoing reasons, it was deemed desirable to improve the design and operation of the controlled rectifier brushless exciter.